Mixed strategy = unprofitable
If a firm does not dedicate itself to one of the 3 strategies by Porter, it will almost certainly suffer from poor profitability. Why? Because it will lose both the (many) customers who demand the lowest possible prices and the (fewer but very profitable) customers who are prepared to pay a premium price for the best offering in the market. Furthermore, the corporate structure, culture, reputation, marketing efforts, motivation systems, etc of firms with a mixed strategy are likely to be unfocused and ineffective.